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Aspirin Synthesis and Analysis Revised: 8/13/12
ASPIRIN SYNTHESIS & ANALYSIS
INTRODUCTION
Since ancient times, the bark and leaves of willow trees have been used as a pain killer. The
active component, salicylic acid (SA), can, however, cause stomach upset because its acidity
(pKa = 2.97) can be higher than the pH of the human stomach (pH ~ 4 after digestion is
complete).
Figure 1. Salicylic Acid
Salicylic acid is a diprotic organic acid with two acidic functional groups: a carboxylic acid and
a phenol. (Acidic hydrogen atoms are blue.) In comparison, the monoprotic acetylsalicylic acid
(ASA, aspirin) is less acidic (pKa = 4.57). The reason: an ester has replaced the acidic phenol in
ASA.
Figure 2. Acetylsalicylic Acid (ASA)
The synthesis of ASA from salicylic acid results in the formation of an ester functional group
and, therefore, is called an esterification. The first step in this esterification is to create a
suspension of salicylic acid (a solid at room temperature) in an excess of acetic anhydride (a
liquid at room temperature). Acetic anhydride serves as both a reactant and a solvent. A
catalyst is required for this reaction. Phosphoric acid, H3PO4, donates a H+ which binds to the
OH
C
O
OH
O
C
O
OH
C
CH3
O
Aspirin Synthesis and Analysis Revised: 8/13/12
reaction complex. As a catalyst, H+ is regenerated (not consumed) by the end of the reaction. As
the reaction proceeds, the solid salicylic acid disappears and the acetylsalicylic acid product
remains dissolved in the hot solution. Once all solid has disappeared (all the salicylic acid has
been consumed) the reaction is complete.
Figure 3. Formation of Acetylsalicylic Acid (ASA, a.k.a., Aspirin)
At this point the excess acetic anhydride must be hydrolyzed (split apart by the addition of
water) to acetic acid. Acetic anhydride is very reactive toward water, so the hydrolysis must be
done slowly – water should be added drop-wise.
Figure 4. Hydrolysis of Acetic Anhydride
More water is then added and the flask is placed in an ice bath to lower the solubility and
precipitate the ASA product. The crude product is then collected by filtration and recrystallized
to further purify the product.
The product’s purity is analyzed 3 ways:
• Melting point is a physical property inherent to a substance’s identity. Point is actually a
misnomer, substances actually melt over a range. The purer the substance, the smaller the
melting range. The broader range of an impure sample (a mixture) results from the creation
of a solution upon melting. A mixture’s melting point is a colligative property, and is,
therefore, dependent on the ratio of chemicals present. For this experiment, assume ASA is a
solvent and any impurities play the role of solute. The more impurities, the lower the melting
point. (Traditionally, this phenomenon is described as freezing point depression, but in this
OH
C
O
OH +
H3C O CH3
O O
O
C
O
OH
C
CH3
O
+
H3CC
OH
OH+, catalyst
50 oC
+
H3C O CH3
O O2
H3CC
OH
OH2O
Aspirin Synthesis and Analysis Revised: 8/13/12
experiment the solvent, ASA, is a solid at room temperature and is heated to melt… not
cooled to freeze.) In this experiment, you will measure the melting point of your crude
product, your recrystallized product, a crushed aspirin tablet, and pure ASA from a
manufacturer.
• Samples with unreacted salicylic acid complex with Fe3+ to create a purple complex in
aqueous solution. Pure ASA samples will remain colorless.
• Finally, the number of impurities in different samples will be determined by thin layer chromatography. Silica will serve as the stationary phase and an organic solvent will be
the mobile phase. The different compounds in aspirin move up the TLC plate at different
rates based the competing intermolecular interactions of the polar silica plate and the less
polar solvent. More polar compounds adsorb more strongly to the silica and do not move as
far up the TLC plate. Less polar components will favor the solvent and move higher. To
The TLC of the ASA you synthesize in lab will be compared with the TLCs of commercially
prepared ASA, the salicylic acid starting material, and an aspirin tablet.
A commercially prepared aspirin tablet contains mostly ASA, but other components such as
caffeine, buffers, and starch binding agents are also added. How much of an aspirin tablet is
ASA? How can we find out?
A solution of ASA is colorless and, therefore, does not absorb light in the visible region between
400 and 700 nm. After treatment with a strong base, a hydrolyzed and deprotonated ASA
fragment (the conjugate base of salicylic acid, SA) is able to bind with Fe3+ ions in acidic
solution. A solution of the resulting cationic Fe3+-SA complex is deeply colored.
ASA Fe3+-SA
Figure 5. Synthesis of Fe3+-SA
O
COH
O
C O
H3C
O-
CO-
O
-OH Fe3+
H+C
OFe
O
O
O
O
O
O
H HH
H
H
H
H H
+
Aspirin Synthesis and Analysis Revised: 8/13/12
When white light (light containing all wavelengths from 400 to 700 nm) passes through a Fe3+-
SA solution, wavelengths corresponding to green light are strongly absorbed and the solution
appears purple. This absorption is a result of the excitation of valence electrons to higher energy
electron orbitals. Therefore, the intensity of light at the absorbed wavelengths is reduced passing
through solution; the amount of reduction is dependent on the concentration of the absorbing
species and the distance the light travels through the solution (path length). This linear
dependence is known as the Beer-Lambert Law (or "Beer's Law"):
(1) A = ε C l
A = absorbance (no units) ε = molar absorptivity coefficient (units = L/mol-cm) C = concentration of absorbing species (units = mol/L) l = path length (units = cm)
If the optical path length and molar absorptivity coefficient are held constant in an experiment,
the absorbance varies with concentration alone. A plot of absorbance vs. concentration is known
as a "Beer's Law Plot".
SAFETY PRECAUTIONS
Safety goggles and aprons must be worn at all times. Acetic anhydride (a liquid at room
temperature) is violently reactive toward water, flammable, corrosive, and can cause burns; add
water slowly, prevent contact with eyes, skin, and clothing. Work with acetic anhydride in the
fume hood only. Wash affected areas thoroughly with cold water. Similar care should be taken
with Fe3+ solution (which has been acidified to pH < 2 with hydrochloric acid). Sodium
hydroxide is corrosive and can cause burns; prevent contact with eyes, skin and clothing. Wash
affected areas thoroughly with cold water. The aspirin tablets used as unknowns may be
contaminated through student activities and are NOT for internal use. When using a pipet,
always use a pipet bulb to provide suction, never pipet by mouth.
Aspirin Synthesis and Analysis Revised: 8/13/12
PROCEDURE
This is a 2 week experiment. Part A and Part C must each be completed in a single laboratory
session. If Part C is done first, save a small amount of the aspirin tablet for Part B. Part B’s
work can be split between the two weeks. Plan your work accordingly.
Part A. Synthesis. Work in pairs.
1. In the fume hood and wearing gloves, combine ~ 1.5 g salicylic acid (record an accurate
mass) and 3.5 mL acetic anhydride (from a graduated cylinder) into a DRY 100 mL
Erlenmeyer flask. Add 5 drops H3PO4 to the suspension.
2. Gently stir and heat the mixture in a 50 °C water bath until all solid disappears (about 10
minutes).
3. Remove from the water bath and carefully add 15 drops distilled H2O. Wait one minute. If
precipitation does not begin add 15 more drops of H2O. If precipitation does not occur after
5 minutes, go on to the next step.
4. Once solid begins precipitating out of solution, add 25 mL distilled H2O. The flask and its
contents may now be removed from the fume hood. Break up any solids and cool in ice bath
for 10 minutes.
5. To complete this step, you must have read the document titled Vacuum Filtration in the
Reading column next to this experiment on the lab manual website. Record the mass of a
piece of filter paper. Collect the precipitate by vacuum filtration and wash with 2 mL
chilled distilled H2O. Discard the filtrate in the sink.
6. Dry for 15 minutes on the filter, using a spatula to loosen and evenly distribute the
precipitate. Record the mass of the precipitate with the filter paper. Save a small amount
(~0.1 g) of this crude product (Sample A) for analysis.
7. Place the rest in a 50 mL beaker. Add 2.5 mL warm ethanol (not boiling). Stir until all solid
is dissolved. Add up to 1.5 mL more ethanol if solids are still present. (Take only the
ethanol needed – no more than 5 mL – (avoid waste). Do not return unused ethanol to the
bottle in the hood.) Add 5 mL warm distilled H2O, allow the solution to cool to room
temperature, and then place in ice-bath for at least 10 minutes.
8. Use vacuum filtration again to collect the crystals (Sample B) on a preweighed filter paper.
Aspirin Synthesis and Analysis Revised: 8/13/12
Discard the filtrate in the sink. Dry the crystals for 15 minutes in the filter and then place
them on a labeled preweighed watch glass in the oven for at least 10 minutes. You should
check the temperature on the oven… if the temperature exceeds 65 °C, your product will
decompose. (Note: While waiting for your product to dry in the oven, continue on with the
next sections of the procedure.)
9. Remove from the oven. Once cooled to room temperature, accurately record the mass.
Collect sample in a vial labeled with the contents (ASA), your name, day and time of you lab
section, date (with the year), and the page numbers of your duplicate lab notebook where the
experiment can be found.
Part B. Analysis. Work alone. Collect all data from this part into one table. In this section you
are comparing the ASA you just synthesized to commercially prepared ASA, salicylic acid
reagent, and the contents of an aspirin tablet.
1. Take melting points: Samples A & B, aspirin tablet, and pure ASA from the manufacturer.
a. Turn on the Mel-Temp. Check out a thermometer from the stockroom (the thermometers
in lab do not go up to a high enough temperature).
b. Weigh and record the mass of an aspirin tablet to the nearest milligram. Crush aspirin
tablet into a powder with a mortar and pestle. Be sure to keep this sample safe, you will
need it in Part C.
c. For each sample, fill a small capillary tube with 2-3 mm of solid.
d. Keeping track of your samples, insert them into the Mel-Temp and monitor them,
recording the temperatures at which melting begins and ends (i.e., the melting
temperature range).
2. Perform the Fe3+ test:
a. Obtain 4 test tubes and label them: Sample A, Sample B, aspirin tablet, pure ASA.
b. Place ~1 mL distilled H2O into each test tube
c. Add a crystal or “pinch” of each sample to its test tube
d. Add 1 drop FeCl3(aq) solution to each test tube.
e. Record your observations. Solutions should be discarded of in the hood labeled “Metal
Containing Waste”.
Aspirin Synthesis and Analysis Revised: 8/13/12
3. Thin Layer Chromatography (TLC):
Chromatography is an analytical method of separating a mixture into its molecular
components. In thin layer chromatography (TLC), a glass or plastic plate with either silica or
alumina adsorbed onto them is used. A capillary tube is
used to apply a small sample (usually in the ng-µg scale) of
the mixture onto the plate. The TLC plate is then placed in
a developing jar containing a small amount of eluent
(solvent). The solvent climbs up the silica or alumina on
the plate and moves the pigment molecules of the mixture
along at different rates. The greater the attraction between
the molecules and the absorbing medium (coated on the plate), the slower the molecules ascend
the coating. The greater the solubility of the components in the solvent, the greater the distance
the molecules move.
1) Place a piece of 5.5 cm filter paper into a TLC developing jar. Pour ~5 mL of ethyl acetate
into the jar (should be 0.5 cm from bottom of jar) and close with a cap. (Take only the ethyl
acetate you need and do not return unused solvent to the bottles in the hood.)
2) Prepare the TLC plate by lightly drawing a line on the plate 1 cm from the bottom with a
pencil. Be careful not to scratch the silica surface. This is the origin line.
3) Place 5 drops of ethyl acetate into a small beaker or vial. Dissolve as much of Sample A into
the ethyl acetate as possible. If the ethyl acetate evaporates before the next step or you put in
too much solid, add the minimal number of drops required to redissolve all solid. (Note: The
objective of this step is to create a very concentrated solution to put onto the TLC plate, but a
lot of solution is not needed and you don’t want to waste sample.)
Aspirin Synthesis and Analysis Revised: 8/13/12
4) Repeat the above step for Sample B, the aspirin tablet, and the pure ASA.
5) Your TA will demonstrate how to use TLC spotter. Use a new TLC spotter for each sample
to transfer it drop by drop to the origin line on the plate. The spots should be evenly spaced
and the right size so that samples will not run together. (Make the spots as small as possible.
Use a UV lamp to see if the spots are of the right size (~1mm diameter).) Do not look
directly into the UV lamp.
Note: Dispose of TLC spotters in the Broken Glass Containers. Do not throw them away in
the trashcans. This is a health hazard for the janitorial staff.
Figure 6. TLC Plate
6) Let the TLC plate dry for 1 minute, then place in the TLC chamber and cap. Make sure your
TLC plate does not touch the filter paper. The purpose of the filter paper is to ensure that the
atmosphere inside the chamber is saturated. If the filter paper touches the side of the TLC
plate, the spots will be drawn sideway.
7) Solvent will begin to move up the plate. Once the ‘solvent front’ is within 1 cm of the top of
the plate, remove the plate from the chamber and mark the solvent front line with a pencil.
This distance from starting line to the final solvent front is needed to determine the Rf value of
Origin: dashed pencil line
Spots for Sample A & B, aspirin tablet, & pure ASA
TLC spotter or capillary:very small
narrow glass tube
Uncoated, bare glass at topUse to handle slide
Silica (white powder) coated area of slide
Analyte solution to spot on plate
Aspirin Synthesis and Analysis Revised: 8/13/12
each component. Note: Rf value cannot be determined if the solvent front could not be seen.
Do not let your solvent front travel to the top of the TLC plate.
8) Circle any visible spots on the TLC plate with a pencil, then use a UV lamp to see spots not
otherwise visible. Measure the distance from the origin to the solvent front and from the
origin to each spot. Sketch or take a digital picture of the plate to include with your report.
Record data neatly in a table and use the following equation to determine the Rf values of the
various spots on the TLC plate:
Rf = Distance from the start to the center of the TLC spot (cm) Distance from the start to the solvent front (cm)
9) Liquids should be discarded in the containers in the hood labeled “Organic Waste”. Used
TLC plates should be discarded in the containers in the hood labeled “Solid Waste”. Turn in
the product for Part A to your TA.
Part C: Analysis II. Creation of a Beer’s Law Plot. Work in pairs.
1. Obtain six sealed cuvettes containing the standard solutions and blank. The blank (labeled
“B”) contains a clear yellow aqueous solution of acidic FeCl3. The remaining cuvettes
(labeled #1-5) contain standard solutions of Fe3+-SA prepared by the stockroom in the
following manner:
a) A stock solution was prepared by dissolving exactly 0.4000 g of pure ASA in
approximately 10 mL of NaOH(aq) and heating the solution to a gentle boil. After
cooling to room temperature, the solution was poured into a 250-mL volumetric flask
and diluted to the mark with DI water.
b) The standard solutions (#1-5) were prepared by transferring exactly 1.00 mL, 2.00 mL,
3.00 mL, 4.00 mL, and 5.00 mL, respectively, of stock solution to clean 100-mL vol
flasks, diluting to the mark with FeCl3 solution, and then filling the appropriately labeled
cuvette.
c) Before coming to lab, calculate the concentration of ASA in cuvettes #1-5.
Aspirin Synthesis and Analysis Revised: 8/13/12
2. Obtain a VIS-NIR spectrometer from the stockroom. Use the USB cable to connect the VIS-
NIR Spectrometer to the computer. Open Logger Pro. The spectrometer should
automatically be recognized by the Logger Pro program.
3. Calibrate the spectrometer by choosing Calibrate → Spectrometer from the Experiment
menu. The calibration dialog box will display the message: “Waiting….seconds for lamp to
warm up.” (The minimum warm up time is one minute.) Note: For best results, allow the
spectrometer to warm up for at least three minutes. Insert the blank (containing
FeCl3(aq)) cuvette (labeled “B”) in the sample compartment. Click OK.
4. Wipe the outside of the cuvette containing solution #5, place in the cuvette holder of the
Spectrometer and click ►Collect. Click ■ Stop once the data collection is complete.
Examine the graph and note the wavelength region of maximum absorbance. (If the graph is
not visible, right click on the plot region and enter 380 nm to 650 nm for the wavelength
region). Remove the rainbow background spectrum by right clicking on the spectrum, go to
graph options, and uncheck "Draw Visible Spectrum". Print the spectrum and data table.
Choose Store Latest Run from the Experiment menu.
5. Click on the Configure Spectrometer Data Collection icon. Under Set Collection Mode,
click Abs vs. Concentration. (Make sure to manually select the wavelength of the
maximum absorbance found in step 4.) Click OK.
6. Insert cuvette #1, click Collect then click Keep. Type in your calculated value for the
concentration of Fe-SA. Repeat with all remaining solutions.
7. Click Stop once the absorbance values for all the standard solutions have been collected.
Title the plot and label the axes before printing.
Absorbance of Fe-SA from an Aspirin Tablet. Work alone.
1. Transfer the remainder of the crushed tablet from Part B to a clean 125-mL Erlenmeyer flask.
Add 10-mL of 1M NaOH(aq) and heat the mixture to a gentle boil. Rinse the walls of the
flask with DI water to recover any hydrolyzed SA splattered on the walls. (Caution - hot
glassware.)
2. Once the solution has cooled to room temperature, transfer it to a 100-mL vol. flask. Wash
the walls of the Erlenmeyer flask with DI water, adding the washings to the volumetric flask.
Aspirin Synthesis and Analysis Revised: 8/13/12
Fill the volumetric flask to the mark with DI water. Cap the flask and invert several times to
mix the solution thoroughly. Allow any solid material to settle to the bottom of the flask.
3. Using the diluted solution from the 100-mL volumetric flask, rinse a 1-mL vol. pipet with a
few drops and then pipet exactly 1.00-mL into a 50-mL volumetric flask. (Take care not to
pipet any solids.) Fill the 50-mL volumetric flask to the mark with acidic FeCl3 solution.
Cap and invert to mix solution. The solution should be a purple color due to the Fe3+-SA ion
formed.
4. Rinse a clean, empty cuvette with a few drops of the unknown solution and then fill the
cuvette about 2/3 full with the solution. Record the absorbance for your unknown at λmax.
5. When finished, pour the solutions in your two flasks and your cuvette into the designated
waste containers in the hood labeled “Metal Containing Waste”. (Return the original 6
cuvettes to the stockroom with your spectrometer.)
Aspirin Synthesis and Analysis Revised: 8/13/12
CALCULATIONS
Part A. Synthesis 1. Calculate the percent yield of ASA for sample B.
2. How much excess reagent (in mL) is left over before the addition of water?
Part B. Analysis I
1. Did the recrystallization result in more pure ASA? Use all observations/results from Part B.
2. Determine the Rf for all of the components using on each slide. What is the Rf of ASA?
Rf = Distance from the origin to the center of the spot (cm) Distance from the origin to the solvent front (cm)
Part C. Analysis II
1. Using Excel, create a Beer’s Law plot (absorbance vs. concentration).
2. What is the value of the y-intercept on your Beer's Law plot? Provide possible explanations
for why this number is not zero. (Take into account the sign of the value.)
3. Using data from your Beer’s Law Plot, what is the value of the molar extinction coefficient,
ε, (also called the molar absorptivity coefficient) for the Fe-SA ion? (The length of the
cuvette is 1.30 cm.) Find the mean value and standard deviation for ε.
4. Using the Beer's Law plot, find the concentration of ASA in your unknown cuvette
graphically and mathematically.
5. The drawing below shows the steps for the preparation of the Fe-SA solution from the aspirin
tablet. You need to think through these steps in order to find the mass % ASA in aspirin.
Aspirin Synthesis and Analysis Revised: 8/13/12
a. What is container 3?
b. What are the volumes of containers 1-4?
c. What is step B called? What volumes are known in the equation underneath the arrow
of step B? What are their values?
d. In which container does a chemical reaction occur? What is that reaction?
e. What is done with the solution in container 4? What information is gained? How is it
gained?
f. Label containers 2-5 with either Mconc or Mdil.
g. Which container(s) contain(s) all of the aspirin tablet?
6. Using the data from your Beer’s Law Plot, calculate….
a. The exact mass (in grams) of ASA in your aspirin unknown.
b. The mass percent of ASA in your aspirin unknown.
QUALITATIVE ERROR ANALYSIS
1. What modifications could be made to the procedure to better account for random
(indeterminate) errors?
2. List three potential systematic (instrumental, methodological, or personal) errors that could
be made in this experiment. (Note: Be specific, systematic errors are in the details. For
example, losing your solution because you knocked over the cuvette is not a systematic error
– it’s a gross one.)
3. Did any gross errors occur? Did you mess up? Did the equipment or instrumentation fail?